The present invention relates generally to the field of printed circuit fabrication, and more particularly to an improved plating buss on a substrate such as for example a printed circuit board (PCB), which addresses manufacturing variations and alignment problems associated with PCB singulation for improved board yield.
Basically, a printed circuit board consists of a sheet of rigid insulating substrate such as phenolic, glass impregnated epoxy or the like, having a pre-defined pattern of thin metallic—usually copper—foil conductive paths (so-called “traces”) appearing on one or both sides of the substrate. These traces collectively define all the electrical interconnections among all the components and are routed between appropriate locations on the board.
Electrolytic plating is one method used to improve electrical conductivity and/or wire bondability in the trace termination areas provided on a multi-layered printed circuit board (PCB). Generally, a panel populated with an array of PCBs is connected to one terminal of either a d.c. or a pulsed plating voltage source and placed in an electrolyte in order to be plated. A metal to be deposited is then connected to the other terminal and similarly immersed in the electrolyte. The transfer of the metal is accomplished via the ions contained in the current flowing between the metal and the panel.
Since electroplating of metals requires that all sites on the panel to be plated must be electrically connected to the plating bath, one prior art method electrically connects the traces to a common straight-line plating buss (also known as a “tie” or “commoning” bar) for convenience. The straight-line plating buss is then used to provide the current during the plating process. However, a plating buss serves no useful function after the electroplating process. Thus, for improved process efficiency, prior art methods have provided the straight-line plating buss centered between adjacent PCBs defined on the panel, such that during a PCB singulation stage, the straight-line plating buss gets cut away.
PCB singulation is the process of taking a finished panel, and separating or depaneling the plurality of PCBs formed thereon into individual PCBs for a subsequent packaging process. One method of PCB singulation is to use a dicing saw. The panel is mounted on a saw carrier, mechanically, adhesively or otherwise, as known in the art. The saw carrier is then mounted on a stage of the dicing saw. Typically, the PCBs are arranged in rows and columns on the panel with the periphery of each PCB being rectangular. During the dicing process, the panel is sawn or diced with a rotating blade along a street lying between each of the rows and columns thereof.
Once all cuts associated with mutually parallel streets having one orientation are complete, either the blade is rotated 90° relative to the panel or the panel is rotated 90°, and cuts are made through streets in a direction perpendicular to the initial direction of cut. Since each PCB on a conventional panel has the same size and rectangular configuration, each pass of the saw blade is incrementally indexed one unit (a unit being equal to the distance from one street to the next) in a particular orientation of the panel. As such, the saw and the software controlling it are designed to provide uniform and precise indexing in fixed increments across the surface of the panel.
As mentioned previously above, PCB singulation is also used to remove process remnants, such as straight-line plating busses used in the (post formation) plating process of the PCBs. An illustration of one prior art plating buss is shown in FIG. 1, wherein a PCB panel 100 has a plurality of traces 102 formed thereon by electrodeposition, which are connected by an associated straight-line plating buss 104. At the PCB singulation stage, a saw blade dices the panel 100 along a cut, which removes panel material between parallel lines 106 and 108 in order to separate adjacent PCB segments 110. If the cut is properly aligned, parallel lines 106 and 108 will flank a street 112 defined between the adjacent PCB segments 112 in order to also remove the straight-line plating buss 104 which was formed there along. As can be imaged, dicing the panel along the street 112 will thereby disconnect the associated traces 102. According the width on the straight-line plating buss 104 can be considered as the “process window” for the PCB singulation stage.
However, over time, the blade and/or stage of the dicing saw may experience drift due to indexing errors of its drive motor and associated gearing. Additionally, variations in the PCB manufacturing process effecting PCB sizes and street locations, and variations in blade width due to uneven heating and wear can also result in indexing errors. Such indexing errors result potentially in the location of cut, illustrated by parallel lines 106 and 108, moving off the street 112 and out of the process window. As illustrated by FIG. 2, moving off the street 112 creates the potential for shorted circuits due to the cut failing to remove the straight-line plating buss 104 connecting the traces 102 in one of the adjacent PCB segments 110. This circumstance decreasing board yield by rejecting PCBs with shorted traces even though all other features on each PCB may be good.